As anyone involved in boating on either a commercial or recreational level knows, the rapidly turning propellers which provide propulsion for nearly every boat pose a constant threat of serious bodily injury. Unfortunately, no other methods or propulsion, e.g. jet propulsion, are as efficient or readily adaptable to the variety of power demands created by marine vessels.
In order to prevent the blades of a propeller from coming into contact with another object, particularly a human or an animal, a substantially cylindrical shroud can be provided. Such a shroud may be attached to the engine housing, lower drive or hull, with the propeller spinning freely therein. Alternatively, the shroud may be integral with the propeller itself and rotate alone with the propeller. This latter concept has been used to provide safer radiator fans, as found in most automobiles.
Marine propellers having a ring or a ring-like structure securely attached to its outer edges so that it spins with the propeller are known. U.S. Pat. No. 1,092,960--Taylor discloses a marine propeller having a solid annular rim encircling the blades and connected thereto at the outer edges thereof. In cross-section, the solid annular rim is symmetrically tapered, decreasing in thickness from front to rear. The Taylor patent teaches that the encircling solid rim enables the diameter of the propeller to be reduced while increasing the power.
When a ring is affixed to a conventional propeller, a phenomenon known as vortex shedding is prevented. This phenomenon occurs when, as fluid flows past an object fluid vortices are periodically shed downstream from the restricting object. The ring essentially prevents an apparent component of passive drag. By minimizing the surface area of the ring affixed to the propeller blades, vortex shedding may be substantially reduced or even eliminated.